st/mesa: need to translate clear color according to surface's base format

st/mesa: need to translate clear color according to surface's base format

When clearing a GL_LUMINANCE_ALPHA buffer, for example, we need to convert
the clear color (R,G,B,A) to (R,R,R,A).  We were doing this for texture border
colors but not renderbuffers.  Move the translation function to st_format.c
and share it.

This fixes the piglit fbo-clear-formats test.

NOTE: This is a candidate for the 7.9 and 7.10 branches.